Satellite-based navigation and guidance systems are known. For example, the Global Positioning System (GPS) is a satellite navigation system used for determining one's precise location, by estimating the three-dimensional, global position of a radio receiver. The receiver, which can be hand-held or mounted to a vehicle such as an aircraft, receives coded signals from a number of earth-orbiting satellite transmitters. Each received signal indicates the position of its satellite transmitter and its transmission time, which enables the receiver (using an internal clock) to approximate signal transit times and estimate the distances to the transmitters. These distances are referred to as “pseudoranges.” In practice, a processor associated with the receiver uses at least four of these pseudoranges to estimate the position (e.g., latitude, longitude and altitude) of the receiver and the associated vehicle with a technique known as trilateration. The accuracy of these position solutions depends on certain factors such as, for example, atmospheric conditions and the performance of the individual satellite transmitters. A satellite navigation system similar to the GPS is the Russian-operated Global Navigation Satellite System (GLONASS).
In recent years, the GPS has been extended for use with aircraft during the more critical portions of a flight (e.g., landings). These satellite-based precision landing systems are ground-augmented, differential systems that typically include two-to-four ground-based GPS receivers, a ground-based differential correction processor (DCP), and a correction-data transmitter. These components are located near the aircraft landing areas involved. The ground-based GPS receivers determine sets of pseudoranges based on signals received from at least four satellite transmitters. These pseudorange measurements are forwarded to the ground-based DCP, which uses the pseudoranges and known positions of the ground receivers to produce an error correction factor. The correction-data transmitter transmits the error correction factor to approaching aircraft, which use this correction data to increase the accuracy of the position estimates provided by onboard GPS receivers. A civilian version of such a satellite-based precision landing system is the GPS-based Local Area Augmentation System (LAAS), and a military version is the Joint Precision Approach and Landing System (JPALS).
Essentially, GPS receivers perform two types of measurements. One such measurement is code-based, whereby the receiver tracks the code modulation of the GPS signal to determine the pseudorange. The other measurement is carrier-based, whereby the receiver tracks the carrier phase of the GPS signal. Notably, phase measurements of the carrier signal typically have much less noise than code-based measurements. Consequently, a carrier phase smoothing process has been developed for use in GPS receivers, which combines the code-based pseudorange measurements with the integral of the carrier phase measurements in order to mitigate the noise inherent in the code-based pseudorange tracking process. Essentially, carrier-smoothing is used in GPS receivers for certain precision applications (e.g., LAAS, JPALS, etc.) in order to eliminate as much high frequency noise as possible from the pseudorange measurements involved.
GPS receivers track the code-modulated signals using delay lock loops (DLLs), and the carrier phase signals are tracked with phase lock loops (PLLs). Carrier-smoothing of the code-based pseudorange measurements is typically performed by coupling data from the carrier phase tracking loops to the code-based tracking portion of the system. Typically, each pseudorange value from the receiver is smoothed with its own smoothing filter. Notably, the Hatch filter is a known smoothing filter that is used in GPS receivers for smoothing code-based pseudorange measurements with continuous carrier phase data.
A significant problem with existing carrier-smoothing filters used in airborne GPS-based precision landing systems (e.g., LAAS, JPALS, etc.) and similar precision applications is that the filters can take up to 5 minutes to stabilize. Consequently, carrier-smoothing of the code-based pseudorange measurements in existing GPS receivers is not available for precision applications until after the smoothing filters stabilize. Thus, for precision position determination applications, the existing GPS receivers are performance limited and essentially unavailable for use for a significant period of time after the smoothing filters are initialized. Note that the period of unavailability is associated with the time constant of the smoothing filter. This association drives designers to use shorter time constants, which degrades the smoothing. Consequently, there is a need for a technique that allows the use of potentially longer time constants without meaningfully degrading availability. Therefore, given the substantive, continuing need to improve the precision and performance of airborne landing systems and similar precision position determination applications, it would be advantageous to provide a system and method that enables an airborne GPS-based precision landing system or similar precision application to begin operating with appropriate performance parameters without having to wait for the carrier-smoothing filters in the GPS receivers to stabilize. As described in detail below, the present invention provides such a system and method.